This invention relates generally to the art of mapping electric fields and, more particularly, to a practical high voltage AC field mapping apparatus and method.
In many applications it is desirable to map the equipotential electric field surrounding high voltage circuit components. In the design of high voltage AC insulating structures, a map of the electric field provides valuable information as to the voltage distribution across the structure. One can determine which portions of the structure will be most highly stressed, permitting the structure to be designed to achieve more uniform voltage distribution and minimize the possibility that any portion of it will fail.
A prior device for mapping high voltage electric fields utilizes a capacitive element placed in the field to determine the potential at a given point. However, the physical size of the device prevents it from being used to map field lines in confined spaces. In the case of high voltage insulators having complex shapes, it is often difficult or impossible to use a large probe to map field lines close to the surface of the insulator. The size of capacitive probes can also limit the spatial accuracy of the readings obtained, since the potential sensed by them is an average potential over the volume of a capacitor.
Another device for mapping high voltage AC fields is a mechanical resonance-type probe, as described in "Resonance AC Potentiometer", Horii et al, Bul. Electrotech. Lab., Vol. 33, No. 9, Japan, 1969. The probe has a small elastic metal strip having a tip which oscillates at a resonant frequency in response to the electric field. The force on the probe tip is proportional to the square of the electrical potential at that location. In the method disclosed, a balancing voltage is applied to the strip to precisely cancel the potential of the electric field at the probe tip and cause the tip to stop moving. The probe can then be moved about in the electric field until the tip stops moving at one or more other locations, defining an equipotential line of the electric field.
In systems of the mechanical resonance type, motion of the probe tip has been monitored by direct observation using a 1400 millimeter telescope. Monitoring has also been accomplished by observing the interference pattern of a laser beam projected onto the probe tip, as disclosed in "AC Probe Potential Measurement", McCann et al, Ontario Hydro Research Division Report, January 1980 However, both techniques require unobstructed line-of-sight light paths, large open areas surrounding the object producing the electric field, and special lighting. In the case of telescope observation the lighting must be very good, while for observation with laser light the level of ambient light must be very low. Telescopes and lasers are also very time-consuming to align due to the relatively large distances involved. The minimum focal distance of the telescope used by Horii et al is approximately thirty feet. Because the mapping procedure requires movement of the probe to a large number of different locations about the sample, the telescope or laser must be repeatedly aligned.
Therefore, in many applications it is desirable to provide a simple and portable device for accurately mapping a high voltage electric field relative to a body of complex geometry.